Added error handling for group already existing (#1902)

restapi/admin_groups.go

@@ -153,7 +153,6 @@ func getAddGroupResponse(session *models.Principal, params *models.AddGroupReque
 	if params == nil {
 		return prepareError(errGroupBodyNotInRequest)
 	}
-
 	mAdmin, err := NewMinioAdminClient(session)
 	if err != nil {
 		return prepareError(err)
@@ -162,6 +161,14 @@ func getAddGroupResponse(session *models.Principal, params *models.AddGroupReque
 	// defining the client to be used
 	adminClient := AdminClient{Client: mAdmin}
 
+	groupList, _ := adminClient.listGroups(ctx)
+
+	for _, b := range groupList {
+		if b == *params.Group {
+			return prepareError(errGroupAlreadyExists)
+		}
+	}
+
 	if err := addGroup(ctx, adminClient, *params.Group, params.Members); err != nil {
 		return prepareError(err)
 	}

restapi/error.go

@@ -43,6 +43,7 @@ var (
 	errBucketNameNotInRequest       = errors.New("error bucket name not in request")
 	errGroupBodyNotInRequest        = errors.New("error group body not in request")
 	errGroupNameNotInRequest        = errors.New("error group name not in request")
+	errGroupAlreadyExists           = errors.New("error group name already in use")
 	errPolicyNameNotInRequest       = errors.New("error policy name not in request")
 	errPolicyBodyNotInRequest       = errors.New("error policy body not in request")
 	errInvalidEncryptionAlgorithm   = errors.New("error invalid encryption algorithm")
@@ -106,6 +107,10 @@ func prepareError(err ...error) *models.Error {
 			errorCode = 400
 			errorMessage = errGroupNameNotInRequest.Error()
 		}
+		if errors.Is(err[0], errGroupAlreadyExists) {
+			errorCode = 400
+			errorMessage = errGroupAlreadyExists.Error()
+		}
 		if errors.Is(err[0], errPolicyNameNotInRequest) {
 			errorCode = 400
 			errorMessage = errPolicyNameNotInRequest.Error()